As I have no interest in pirating the anime to understand the roles will I be more of a hindrance in playing? The rules say it's possible for me to play but I want to also know I'll be effective, considering I'm still a mafia noob.
You can have zero knowledge of the series and still play well (See: pretty much everyone in the Battlestar Galactica mafia and a bunch of people in the Resident Evil mafia), as long as you understand the roles' interactions to some extent. As the host I'll clarify anything that you don't understand if you ask.
"Offline players" refers to people not playing the game, correct? They can be offline from SEN, but they can't be "offline" from whatever this digital world is, right? Because if we can only vote for people who are at that minute active in the topic on SEN it will be impossible. But if it's just people not playing this rule seems redundant. Are there roles that are considered offline from the game?
"Offline" here is the equivalent of "dead" in other mafia games, while "online" means "living". Being "lynched" is replaced with having your Augmented Reality Visors confiscated and destroyed, which makes the player "go offline". Keeping it G-rated

"Other forms of communication are frowned upon" I want some clarification: how can we communicate? Does all the communication after happen in thread? Is that what "intercom" means? Does anything I tell player A have to be available to player C too? Because it also says I can communicate any way I like, which I think means I could PM one person, or Skype a person, etc. I want to clarify.
As long as you're still alive, you can communicate via thread (making it visible to everyone, which is what "intercom" means), PM, Shoutbox, or whatever else you want.
Once you die, though, you can no longer post in the thread. You are not restricted from using the shoutbox or PMing people since I can't really stop you, but as general etiquette dead players should not try to actively influence the game while dead.
As for as the "last will" goes, does this mean that I can't attempt to hold the town hostage by threatening to reveal their roles this time?

You can write whatever you want in your last will and testament, but keep in mind threatening to reveal roles once you're up on the chopping block actually makes it
more likely for you to be lynched since that's how a mafia member would try to save their ass.
None.